I think both are needed: the availability of native code compilation influences the artifacts I'm going to build and I'd rather have opam tell me that than probe the environment. Maybe it's better to be more abstract than the actual tool (e.g. is `ocamlmklib` available in byte code only ?) and have something like `native` or `native-available` or `native-compilation-available`. 

For this actual variable I would suggest something like `opt-ocaml-tools` or `opt-ocaml-tools-installed`.